Venus Transit FAQ
Q1. What (not on Earth) is a transit?
A1. A Transit is basically one of the many shadow plays that Solar system objects indulge in - In the current case that we are interested in - it will be Venus moving in front of the disk of the Sun.
Coincidentally, Moon and the Sun present almost the same size of a disc as seen from earth, so that Moon covers the Sun entirely, during a total Solar Eclipse, or - what would technically be called an Occultation of the Sun, by the Moon. We do have annular eclipses sometimes, when the Moon's disc is seen, just a little smaller than the Sun's and a maximum eclipse at such times would leave an outer ring of the Sun visible.
Planetary transits are a bit like such annular eclipses, except that the coverage of the Sun's surface is even smaller in this case - such a small dot of the Sun's disk is covered by the Planets Mercury or Venus, when they move in front of the Sun, that there is no noticeable decrease in the amount of sunlight reaching us - one would not even get to know that there is a Planetary transit, unless we know that it is going to take place, and then set out to observe it.
Q2. Can I see Venus Transit with the naked eyes?
A2. Do not ever consider looking at the Sun, without proper filters and protection. Grade 14 of welder’s glasses would be the required filters for protection. Safest way would be to project the image of the Sun, as seen through a Telescope, onto a screen. However, the operative thing, in the case of Venus transit, is that, it is a phenomenon visible without additional optical aids, other than the filters, for safety. Telescopes equipped with suitable solar filters will give a much better view of the transit, of course. Viewing a Transit of Venus, without any other optical aids than suitable filters, may be just on the threshold of visibility - it is likely that those with good eyesight, may be able to see it, while it may not be discernable to every eye.
Q3. Where can the upcoming Venus Transit be seen from?
A3. India is particularly well placed for viewing the entire transit. Europe, many parts of Asia and Africa, are similarly well placed. Australia and eastern United States can see part of the Transit - with Australia and some parts of Asia in the east, missing the later stages of the Transit, while Eastern parts of America, western Africa and the Caribbean would witness these later stages. Southern Chile, Argentina, western parts of America, Hawaii and New Zealand miss out on all the stages of this transit.
Q4. Why is this event so important?
A4. Observations of the Transit of Venus, for the first time, gave accurate estimates of the all important distance scale - The Astronomical Unit - distance between the Earth and the Sun. This distance then, serves as a basis for all other distance determinations in the Universe. The first accurate measurements of this distance happened with the global expeditions during the 18th century transits of Venus. Further refinements came with the 19th century observations. The most accurate measurements of this distance, currently, are determined using other, more modern, techniques.
Q5. Will the June 8, 2004 observations improve our current estimation of the Astronomical Unit?
A5. No, as mentioned above, modern techniques of radar ranging to Venus give the most accurate measurements of Solar System distances. The Transit method cannot improve on that. However, it does give students an unprecedented opportunity of recreating measurements that were at the forefront of Astronomical research, in history.
Q6. What is the frequency of these transits?
A6. First - orbital inclinations. Most of the Solar System orbits do seem to be laid out on a table top - all in a plane. However, there are small (in most cases) angles of these orbits to the orbit of the Earth. Thus, we do not see transits during every orbit of Mercury or Venus, analogous to the absence of eclipses at every Full or a New Moon. It is only when Mercury or Venus are at the crossing points of their orbits with that of the Earth - at the ascending or descending nodes of their orbits, that transits are possible. Transits of Mercury are more frequent, one may have about 13 or 14 transits in a century. Venus, being much slower in its orbit around the Sun, transits very rarely as seen from the Earth. The Transits of Venus have a peculiar periodicity of 8, 121.5, 8 and 101.5 years. The last transit occurred in 1882 - this however was not visible from India. India witnessed a transit of Venus 8 years prior to that, in 1874. The upcoming transits will be in the years 2004 and 2012, following this, the next set of transits will take place in the years 2117 and 2125.
Q7. When were transits first observed?
A7. The first appreciation of the possibilities of transits of the inner planets against the disk of the Sun, seem to be attributable to Kepler. His calculation of the possible transits of Mercury and Venus, in 1631, allowed Gassendi to observe the Mercury Transit, that year. Later, Jeremiah Horrocks, corrected Kepler’s calculations and realized that a Transit of Venus would also occur in 1639. This, he did subsequently observe, from Hoole, near Preston, in England. He also alerted William Crabtree about this transit, who observed it from Salford, making these the first recorded observations of a Transit of Venus.
Q8. What is the relevance of Transits in modern Astronomy?
A8. We cannot better radar ranging measurements, with Transit observations, as far as the Astronomical Unit determination is concerned. However, transits do have a substantial role to play, when we come to detection of extra Solar Planets. Observed transits of extra solar Planets in front of their parent star - observations of a small dip in the light output by the star - form the basis for the direct detection of the presence of Planets around stars.